In a fingerprint acquisition system a finger is placed contact with a flat surface that senses the pattern of ridges and valleys on the finger. Acquisition is often accomplished by optical techniques that provide an electronic image of the fingerprint. For example, the fingerprint may be optically imaged onto a light-sensitive detector such as a chargecoupled device (CCD). Optical techniques have a number of drawbacks, including their relatively high cost, complexity, low image quality, and susceptibility to circumvention by using, for example, a photocopy of a fingerprint. Moreover, the optical system is subject to misalignment and breakage.
Another type of fingerprint acquisition system is disclosed in U.S. Pat. No. 4,353,056. In this system an array of capacitors are formed in a flexible substrate. When a finger comes in contact with the substrate, those capacitors directly under a ridge of the finger will be compressed, altering their capacitance. Those capacitors not under a ridge will not be compressed and hence their capacitances will be substantially unchanged. An image is obtained by measuring the capacitances of the various capacitors in the array. This method has a number of limitations, including its inability to form a quality image because it can only detect the presence or absence of ridges as well as its susceptibility to circumvention by using, for example, a plastic replica of a fingerprint. Additionally, manufacturing may be unduly complex because of the incompatibility between a flexible substrate and conventional integrated circuit fabrication techniques. Moreover, since the system must be flexible it cannot be protected by a resilient inflexible material such as diamond.